The general problem is to furnish a device capable of transmitting in a secure manner a set of high-quality visual films or programs in a format of the MPEG-2 TS (Transport Stream) type directly to a TV screen and/or for being recorded on the hard disk of a box connecting the telecommunication network to the television screen while preserving the audiovisual quality yet avoiding any fraudulent use such as the possibility of making pirated copies of films or audiovisual programs recorded on the hard disk of the decoder box.
It is possible with current solutions to transmit films and audiovisual programs in digital form via broadcasting networks of the microwave, cable, satellite, etc. type or via telecommunication networks of the DSL (Digital Subscriber Line) type or BLR (local radio loop) type or via DAB (Dig-ital Audio Broadcasting) networks. Furthermore, to avoid the pirating of works broadcasted in this manner, they are frequently encrypted by various well known means.
WO 00/60846 proposes a solution for the distribution of video on demand via a system of distributed servers. The protection of the content of the digital video is based on an encryption system based on digital keys. The chief innovation of that disclosure is an encryption and decryption of MPEG-TS streams simplified in that an analysis of the MPEG is carried out beforehand to optimize the encryption as a function of the structure of the stream. The calculating time is in fact sharply reduced due to a selective and optimal encryption of the MPEG-TS stream. Analysis of the stream consists primarily of marking the TS packets containing information essential for a correct decoding of the video in the MPEG sense in such a manner that in a second encryption only the useful data of the marked TS packets are encrypted. Thus, the essential information is unavailable for an unauthorized decoding and the stream can therefore not be displayed, while retaining a reasonable calculating time for the encryption. However, that solution does not permit a resolution of the security problem because the protection of the video stream is based on an encryption system based on digital keys but for which the video stream transmitted to receiver equipment contains in encrypted form all the information initially present in the original video stream. Therefore, the video stream received by the client equipment already contains all the information initially present in the non-encrypted video stream even if they are stored in a form that does not permit their being used. An ill-intentioned user can process the encrypted video stream in such a manner as to put it back in a form that can be used by a classic video decoder and can therefore circumvent the protection system because a direct and correct reconstitution of the audiovisual stream from only the encrypted stream is possible.
A solution to this problem would consist of making the reconstitution of the original video stream from only the protected video stream impossible. Moreover, the modified packets can be detected in such a manner as to be recognized by the decryption module. However, making the encrypted TS packets readily detectable facilitates all the more the task of ill-intentioned individuals who might desire to circumvent this protection. A solution to this problem would consist of not permitting the detection of scrambled packets via a simple analysis of the stream. Therefore, the prior art does not satisfy the technical problem and the criteria for high security constituting subject, matter of the present invention.
To prevent access to a compressed digital copy of a video program, another solution consists of adding the protection to the video when it is still in analog form prior to digital compression. Thus, to remove the associated protection it is necessary to process the video stream after the MPEG decoding since it was in this form that the protection was introduced, which implies that only a decompressed version of the non-protected stream is accessible. Now, an initial, non-compressed version of a video stream is difficult to exploit. In this connection, the general solution disclosed by EP 1 011 269 A1 is of the same type as that of WO 00/60846 in that it is also based on an encryption based on digital keys. However, that solution is insufficient because the reconstituted video is not perfectly identical to the original video.
Another solution consists of preventing the possibility of viewing or making a non-compressed digital copy of the stream by conditioning the use of the video stream on the obtention of protection information from distributing equipment. In this connection, the solution disclosed by EP 0 975 165 A2 (is known that is similar to that of WO 00/60846 in that it makes use of an encryption based on digital keys. However, that solution is insufficient for remedying the technical problem of security because the information sent from the distributing equipment to the addressed equipment for the use of the video stream is information connected to the limited reproduction rights of the video stream. Thus, the video stream received by the client equipment also contains all the information initially presented in the non-protected video stream.